addresses GH #21646

[elmq0022]

• closes Test fixture datapath uses relative instead of absoulte file paths #21646
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[jreback]

this fixes the local running issue?

[elmq0022]

This allows pytest to be called from any directory within the project and successfully run tests using the datapath fixture.

[codecov]

Codecov Report

Merging #21647 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #21647   +/-   ##
=======================================
  Coverage    91.9%    91.9%           
=======================================
  Files         154      154           
  Lines       49558    49558           
=======================================
  Hits        45545    45545           
  Misses       4013     4013

Flag	Coverage Δ
#multiple	90.27% <ø> (ø)	⬆️
#single	42.02% <ø> (ø)	⬆️

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[TomAugspurger]

Thanks!